1. Field of the Invention
The present invention relates to an image sensor. More particularly, the present invention relates to an image sensor and a method for manufacturing the same, capable of improving the sensitivity of a photodiode by making a rough surface on the photodiode such that the light absorption coefficient of the photodiode can be enhanced and light reflected from the photodiode can be prevented from being discharged through an upper layer.
2. Description of the Related Art
In general, image sensors are semiconductor devices for converting optical images into electric signals, and are generally classified into a charge coupled device (CCD) image sensor and a complementary metal oxide semiconductor (CMOS) image sensor.
The CMOS image sensor includes aphotodiode for detecting irradiated light and a logic circuit for converting detected light into electric signals which can be processed (e.g., as data). As the quantity of light received in the photodiode increases, the photosensitivity of the image sensor is improved.
To improve the photosensitivity, either a fill factor, which is a ratio of a photodiode area to the whole area of the image sensor pixel, must be increased, or a photo-gathering technology is used to change the path of light incident onto an area excluding the photodiode area such that the light can be gathered in the photodiode.
A representative example of the photo-gathering technology is to make a micro-lens. In other words, a convex micro-lens is formed over the photodiode using a material having good light transmittance characteristics, thereby refracting the path of incident light in such a manner that a greater amount of light can be transmitted into the photo-diode area than in the absence of the microlens.
In this case, light parallel to an optical axis of the micro-lens is refracted by the micro-lens so that the light is focused on a certain position of the optical axis.
Meanwhile, a typical image sensor includes a photodiode, an inter-layer dielectric layer, a color filter, and a microlens.
The photodiode detects light and converts the detected light into electrical signals. The inter-layer dielectric layer insulates adjacent metal interconnections from each other. The color filter filters white light to one of three primary colors (e.g., red, green or blue [RGB], or alternatively, yellow, cyan or magenta [YCM]). The micro-lens condenses light on the photodiode.
Hereinafter, a conventional image sensor and a method for manufacturing the same will be described in detail with reference to accompanying drawings.
FIG. 1 is a schematic view showing the conventional image sensor. FIGS. 2A to 2C are sectional views showing the method for manufacturing the conventional image sensor. FIG. 3 is a view showing the path of light passing through a micro lens in order to explain problems of the conventional technology.
As shown in FIG. 1, an inter-layer dielectric layer 20 is formed on a semiconductor substrate 10 formed with a plurality of photodiodes 40, and an RGB color filter layer 30 is formed on the inter-layer dielectric layer 20 such that the RGB color filter layer 30 corresponds to the photodiodes 40.
In addition, a planarization layer 25 is formed on the color filter layer 30 in order to planarize an irregular surface of the color filter layer 30, and microlenses 50 are formed on the planarization layer 25 such that the microlenses 50 correspond to the photodiodes 40 and the color filter layer 30.
In this case, the microlenses 50 are patterned and formed in the shape of convex lenses in order to condense light on the photodiodes. To this end, a photolithography process is employed.
In detail, as shown in FIG. 2A, after coating photoresist 60, which is a material for the micro-lens, on the planarization layer 25, the resultant structure is covered with a mask 61, and then an exposure process is performed using a defocus phenomenon, thereby patterning the photoresist 60 in the shape of a trapezoid after development, as shown in FIG. 2B.
Thereafter, as shown in FIG. 2C, the trapezoidal photoresist pattern is heated to the melting point to reflow. When the reflow process is performed, the photoresist pattern is rounded with flexibility, thereby completely forming the micro lenses 50.
Accordingly, when light is emitted from a predetermined object 70, the light incident on the image sensor through the micro-lens 50 is refracted and focused on the photodiode 40 as shown in FIG. 3.
However, the conventional photodiode does not absorb all of the light, but reflects some of the light. The reflected light may degrade the sensitivity of an image sensor.